@function foo( $bar)
  @return $bar


@function baz()
  @return false



=bar ($baz )
  content: $baz


.foo
  +bar( 'Hello' )
  content: foo('bar')
  width: calc( 100% - 10px)

// Top level mixin
+hello(
  $foo,
  $bar,
  $baz
)

.foo
  // Nested Mixin
  +hello(
    $foo,
    $bar,
    $baz
  )

  // CSS built-in function
  background: transparent linear-gradient(
    to bottom,
    #ff0000 0%,
    #00ff00 40%,
    #0000ff 40%
  )

  // User-defined function
  content: goodbye(
    $foo,
    $bar
  )
